1. Field of the Invention
The present invention relates to an insulating film used as a tunnel oxide film of a flash memory, a gate oxide film of a transistor or the like, and to a method of forming such a film.
2. Description of the Related Art
Apart from the memory for general use, such as DRAM, SRAM or the like, recently there has been an increasing demand of EEPROM (flash memory), in which data remains if the power is turned off, data can be rewritten for 106 times or more, and which is suitable for a large increase in capacity. The reliably of such a flash memory is deeply correlated with the reliability of the silicon oxide film (insulating film) having a thickness of 10 nm or less, which is called a tunnel oxide film. Thus, the reliability of the flash memory is greatly influenced by three different electrical characteristics: (1) dielectric breakdown life, (2) charge trap amount and (3) stress leak amount, which are items of evaluating the reliability of a silicon oxide film. If at least one of these characteristics is deteriorated, the device cannot function sufficiently.
As described above, it is necessary for a flash memory to keep up the above three reliabilities at the same time; however, with a conventional thermal oxide film, all of the reliabilities cannot be satisfied. For example, as to the conventional thermal oxide film, there has been a report of achieving the prolongation of the dielectric breakdown life and the reduction of the charge trap amount; however, there has been no report of achieving a significant reduction in the stress leak amount.
Further, in the case of the gate oxide film of a transistor, for example, the variance of the element characteristics occurs due to hot carrier implantation. The hot carrier implantation is a phenomenon in which electrons in a channel of a transistor becomes hot as energy being supplied from an electric field in the direction along the channel, and the electrons are implanted into the gate oxide film. This phenomenon generates a charge trap in an oxide film, or an interface state at an interface between the oxide film and the substrates, which causes a variation in the threshold value of the transistor or a gm deterioration.
Meanwhile, with regard to the method of forming a tunnel oxide film, in the case where an oxide film having a thickness of about 5 to 10 nm is formed by a conventional vertical diffusion furnace, it has been reported that an oxide film, which was oxidized in a water vapor atmosphere, in a so-called wet oxidizing atmosphere, has an insulation breakage life longer than an oxide film, which was oxidized at a temperature of 850xc2x0, in a so-called dry oxidizing atmosphere (dry oxidizing atmosphere). Further, with regard to the amount of stress leak generated, it has been reported that the amount of the stress leak of an oxide film formed in a water vapor oxidizing atmosphere is suppressed more (lower) than the case of an oxide film formed in a dry oxygen atmosphere. Meanwhile, it has been reported that the density of the charge trap is reduced more in an oxide film formed in a dry oxygen atmosphere than in an oxide film formed in a water vapor atmosphere. However, there has been no report on a silicon thermal oxide film or a method of forming such a film, which satisfies all of the above-described three reliabilities at the same time.
As described, with a silicon thermal oxide film, by itself, it is not possible to satisfy the three reliabilities at the same time, and therefore a silicon oxynitride film in which nitrogen is introduced into a silicon thermal oxide film, is presently used as a tunnel oxide film. The silicon oxynitride film is able to decrease the stress leak amount, and further satisfies the conditions of a dielectric breakage life which the silicon thermal oxide film itself possesses and a decrease in the charge trap amount. Thus, the silicon oxynitride film satisfies the above-described reliabilities at the same time.
However, the silicon oxynitride film has current-voltage (I-V) characteristics different from those of a silicon oxide film which does not contain nitrogen, since nitrogen is introduced to a silicon thermal oxide film. Also, the device operation characteristics are varied along with the amount of nitrogen introduced, which creates a problem. As typically exemplified in the thinning of a thermal oxide film at an element separation edge, the three reliabilities are very much influenced by the reliability of the silicon thermal oxide film itself, which serves as an underlayer. In reality, it is necessary to improve the film quality of the silicon thermal oxide film must be further improved in terms of all of the three reliabilities, from those of the present status.
The present invention has been proposed in consideration of the above-described drawback of the prior art technique, and the object thereof is to provide an insulating film having a long dielectric breakage life, capable of reducing the amount of the interface state and the amount of charge trap generated, and capable of remarkably reducing the amount of stress leak generated when a high electrical field stress is applied, as well as a method of manufacturing such a film.
According to an aspect of the present invention, there is provided an insulating film formed on a surface of a substrate and made of a material containing oxygen, wherein a charge correction is carried out at a 1s peak position of a carbon adsorbed on a surface of said insulating film, and relative amounts between first to fourth peaks obtained when an oxygen 1s peak of said insulating film is decomposed by a same half width of 1.208 eV into a first peak at the oxygen 1s peak position obtained from an xcex1-quartz crystal charge corrected similarly, and second to fourth peaks at positions of +0.87 eV, xe2x88x920.35 eV and xe2x88x920.83 eV, respectively from the oxygen is first peak position, have relationship of that the third peak is higher than the second and fourth peaks, and the first peak is higher than the third peak, when a portion about 1 nm thick from the surface of the substrate of the insulating film is analyzed by a photoelectronic spectral method for an photoelectron extracting angle of 15xc2x0 or less. With this structure, it becomes possible to form an insulating film having excellent electrical characteristics, even if the thickness thereof is only 3 nm or more and 10 nm or less.
With regard to the insulating film of the present invention, the insulating film made of a material containing oxygen, is meant to be a silicon oxide film, silicon oxynitride film or the like.
Further, with the insulating film of the present invention, the portion about 1 nm thick (deep) from the surface of the substrate is analyzed by a photoelectronic spectral method while setting the photoelectron extracting angle at 15xc2x0 or less. This is because the difference from a conventional insulating film can be observed only in a region very close to the surface of the substrate, as the matter of the sensitivity of the analysis. Therefore, for example, at a section 10 nm-deep from the surface of the substrate, a fine difference between the insulating film of the present invention and a conventional one cannot be observed due to a low sensitivity of the analyzing device. Further, the reason why the photoelectron extracting angle is set at 15xc2x0 or less in the photoelectronic spectral method, is to analyze the insulating film mainly. When the angle exceeds 15xc2x0, for example, when the angle is 90xc2x0, the amount of photoelectrons from the substrate increases, and therefore the sensitivity for analyzing the insulating film is relatively decreased.
In the insulating film of the present invention, it is preferable that the relative amount of the first peak should be 40% or more of an area of the oxygen 1s peak, the relative amount of the third peak should be 30% or less of the area of the oxygen 1s peak, and the relative amounts of the second and fourth peaks should be 20% or less of the area of the oxygen 1s peak. This is because if the relative amount of the first peak is less than 40% of the area of the oxygen 1s peak, the reliability of the insulating film is decreased, making a film structure having the same characteristics as those of an insulating film obtained by the conventional method.
According to another aspect of the present invention, there is provided a method of forming an insulating film on a surface of a substrate, comprising the steps of heating the substrate in a processing chamber an atmosphere inside which is a water vapor atmosphere maintained at 900xc2x0 C. or higher; and carrying the substrate from the processing chamber into an inert gas atmosphere, cooling down the substrate at a maximum temperature decreasing rate of 15xc2x0 C./sec or more to cool the substrate to a temperature of 600xc2x0 C., and forming an insulating film or a surface of said substrate.
According to the present invention, there is further provided a method of forming an insulating film on a surface of a substrate, comprising the steps of introducing water vapor into a processing chamber in a state of maintaining a substrate at a particular temperature of 100xc2x0 C. to 600xc2x0 C. within the processing chamber; heating said substrate at a particular temperature of 900xc2x0 C. or more at a maximum temperature increasing rate of 25xc2x0 C./sec or more; cooling said substrate to a temperature of 600xc2x0 C. or less at a maximum temperature decreasing rate of 15xc2x0 C./sec or more; and forming an insulating film on a surface of said substrate. In this method, it is preferable that the atmosphere consisting of the diluting gas is created in the processing chamber before the temperatures of the processing chamber reaches a set temperature range of a thermal processing temperature, and when the temperatures reach the set temperature range, water vapor should be introduced to the processing chamber along with the diluting gas, thus creating a diluted water vapor atmosphere.
In the method of the present invention, it is preferable that the water vapor should be diluted with an insert gas, in order to form a thin insulating film by a high-temperature reaction. With this operation, an insulating film having a thickness of 10 nm or less can be formed in a short period of time even at a high temperature of 1100xc2x0 C. Further, the dilution should preferably be 1 to 50% in consideration of the processing time for the single wafer process.
It was found that in order to form an insulating film of the present invention by above-mentioned method, there are four important conditions for the formation of a film, that is, the oxide gas atmosphere, the oxidizing temperature for the substrate, the temperature increasing rate and the temperature decreasing rate.
More specifically, in the film forming method of the present invention, the atmosphere in which the substrate is placed is set at about 900xc2x0 C. or higher. This is because if the temperature is lower than about 900xc2x0 C., the amount of stress leak, in particular, is increased, of the three electrical characteristics. Further, the maximum temperature decreasing rate while cooling the substrate down to about 600xc2x0 C. is set about 15xc2x0 C./sec or more. This is because if the maximum temperature decreasing rate is lower than about 15xc2x0 C./sec, the amount of stress leak and the dielectric breakage life, in particular, are deteriorated, of the three electrical characteristics. The maximum temperature increasing rate while heating the substrate up to about 900xc2x0 C. is set about 25xc2x0 C./sec or more. This is because if the maximum temperature increasing rate is lower than about 25xc2x0 C./sec, the thickness of an oxide film (low temperature oxide film) formed at a low temperature of 900xc2x0 C. or less while increasing the temperature, is increased, and as a result, the thickness of an oxide film (high temperature oxide film) formed at a high temperature effectively, is decreased.
In the present invention, as the substrate, a silicon substrate (monocrystal silicon substrate, polysilicon substrate, amorphous silicon substrate) or a substrate having a film made of a silicon material (a monocrystal silicon, polysilicon or amorphous silicon) on the surface thereof, in particular, SOI (silicon on insulator) substrate, and an epitaxial silicon substrate or the like can be used. The insulating film means a gate insulating film, a tunnel insulating film between a substrate and a floating gate, a insulating film at the gate bird""s beak region, and the like. It should be noted that the insulating film of the present invention includes an insulating film for electron conduction, for a conduction band, and an insulating film for hole conduction of a valence band.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.